Wave transmission network



4- s. G. HALE 2,362,549

WAVE TRANSMISSION NETWORK Filed Sept. 25, 1942 FIG.

1N VENTOI? 5.6. HALE 5V Patented Nov. 14, 1944 WAVE TRANSMISSION NETWORK Stuart G. Hale, Chatham, N. 1., aslignor to Bell Telephone Laboratories, Incorporated, New York,N. Y., a corporation of New York Application September 23, 1942, Serial No. 459,401 7 auxiliary coils arepreferably made variable so 31 Claims.

This invention relates to wave transmission networks and more particularly to a network for suppressing undesired ,longitudinal currents in a transmission line.

The principal object is to suppress undesired longitudinal currents over a range of frequencies without interfering with the transmission of signal currents in the same range in the loop circuit of a transmission line.

Another object is to improve the impedance match between a suppression network and the terminal loads between whichit operates, and thereby reduce reflection eflects at the junctions.

In order to suppress undesired longitudinal currents flowing in the same direction in both wires of a transmission line, a so-called suppression coil is inserted in the line. Such a coil has two equal, closely coupled windings connected one in each side of the line. A coil structure which will provide the extremely high balance to ground required and close inductive coupling will also have a comparatively large interwinding .capac itance. across the line and, as a result, the suppression coil will have a poor impedance match with the line. This impedance mismatch causes unde- 'sirable reflection effects at the junctions of the coil and the line.

In accordance with the present invention the impedance match between thesuppression coil and the line is greatly improved by the addition of auxiliary coils. There are preferably three of these and each one comprises two equal windings, one in each side of the line. One of the auxiliary coils is inserted in series at the midpoint of the suppression coil, which is split into two equal sections, and the other auxiliary coils are connected in series at the outer ends of the suppression coilisections. These auxiliary coils and the interwinding shunt capacitances associated with the two sections of the suppression coil thus form a two-section, low-pass wave fllter effective in the loop circuit; The lnductances of the auxiliary coils are so chosen that this fllterr has a nominal image impedance which matches the characteristic impedancepi the line or other terminal load. The impedance match can be extended to a higher frequency if the end sections of the filter are given a fractional termination equal to K, where K in most cases falls between 0.3 and 0.4, and general y has an optimum value in the neighborhood of 0.36. To accomplish this, the inductance of each of the end auxiliary coils is made equal to K times the inductance oi the This capacitance is, in eifect, shunted ends of the suppression coil.

that the image impedance of the network may be precisely adjusted to the desired nominal value.

For a given nominal image impedance, the cut-off frequency of the filter is inversely proportional to the magnitude of the interwinding capacitances associated with the suppression coil. Since the cut-oi! must be above the upper limit of'the range of the signal currents, and preferably several times this frequency, it is desirable to limit these capacitances to the lowest possible values.

The suppression coilis preferably so designed that the longitudinal distributed capacitance The nature of the invention will be more fully understood from the following detailed description and by reference to the accompanying drawing, of which:

Fig. 1 is-a schematic circuit of a suppression network in accordancewith the invention;

Fig. 2 is a schematic circuit 'ofthenetwork effective in the longitudinal circuit; and

Fig. 3 is a schematic circuit of the low-pass fllter efl'ective in the loop circuit. 1

Taking up the figures in more detail, the suppression network shown in Fig. 1 comprises a suppression coil,- made up of two equal sections La and Lt, and three auxiliary coils L1, Lo and L5. Each of the auxiliary coils, and also each section-of the suppression coil, has two equal windings, connected one in each side of the line. The coils are all connected in tandem between two pairs of terminals L2 and 3, 4, one pair of which serves as theinputand the other as the output. The central auxiliary coil Lo is connected between the sections In and L4 of thesuppression coil and the other two auxiliary coils L1 and Le are connected, respectively, at the outer The twovariable capacitors C1 and C2 arebridged across the outer terminals of the suppression coil, one in each side of the line. I l

The function of the. suppression coil is to suppress undesireddongitudinal currents. that currents flowing in the same direction in both central auxiliary coil. The inductances of the u sides of the line, with ground as the return path.

However, this coil must not impede the flow of signal currents in the loop circuit, that is, the metallic circuit comprising the two sides of the line. To accomplish this, the two line windings of each of the sections L2 and L4 of the suppression coil are closely coupled inductively and connected in the series-opposing relationship for the loop circuit, which is parallel-aiding for the longitudlnal circuit. This coil will, then, introduce a maximum impedance in the longitudinal circuit and a negligible impedance in the loop circult.

The network effective in the longitudinal circuit is shown schematically in Fig. 2. The parallel-aiding inductance of the suppression coil provides a, series impedance in this circuit. The auxiliary coils L1, In and L do not appear in this circuit since their inductance is negligibly smail Bridged between theassociated with the suppression coil and are,

therefore, shown in broken line. These capacitances cause the suppression coil to antiresonate at some frequency and the coil is so designed that this frequency is at or near the upper limit of the suppression range, thus providing maximum suppression of longitudinal currents at this frequency. The inductance of the suppression coil is, therefore, made as large as possible consistent with providing antiresonance at the desired frequency. However, since the longitudinal distributed capacitances C3 and C4 are subject to manufacturing variations from coil to coil the variable auxiliary capacitors Cl. and C2 are added so that the frequency of antiresonance may be accurately adjusted. The capacitances of the capacitors Cl and C2 are kept equal so that a high degree of balance to ground may be maintained for the network. These capacitances may conveniently be furnished by short sections of a parallel pair of wires.

In order to get the required high degree of bal ance to ground, together with good magnetic coupling between line windings, the suppression coil may be wound from a twisted. pair conductor in two sections on a toroidal 'core of powdered magnetic material. has a comparatively large shunt capacitance effective between the windings in the two sides of the line. This inherent shunt capacitance de grades the impedance match between the network and the line and gives rise to reflection effects in the loop circuit.

In accordance with the present invention, the detrimental effect of this shunt capacitance is effectively overcome by adding the three seriesconnected auxiliary coils L1, L3 and L5 so as to form, with the shunt capacitance, a low-pass filter structure. Fig. 3 shows the resulting network effectiv in the loop circuit. The capacitance C5 is the total shunt capacitance effective between the two line windings of the suppression coil section L2, and C6 is that associated with the coil section L4. Since these are inherent capacitances they are shown in broken line. The suppression coil furnishes only. a, negligible inductance in the loop circuit since its line windings are Such a construction, however,

closely coupled and connected series-opposing for this circuit. The capacitances C1, C1, C3 and C4 may also be disregarded in this circuit since they are comparatively small and their chief effect is .to produce a peak of attenuation at some high in Fig. '7 of United States Patent No. 1,227,116,

issued May 22, 1917, to G. A. Campbell. In accordance with filter theory the full central coil La will have a series-aiding inductance given by the formula La=C'R 1) where C is the full shunt capacitance and R is, the nominal image impedance of the filter. As is customary, R is made approximately equal to the impedance of the transmission line or other terminal load with which the filter is to operate. Each of the end coils Li and L5 will have a seriesaiding inductance equal to where K is the value of the fractional end section. Each of the shunt capacitances Cs and Cl is made up of a half shunt capacitance 0.5C, associated with the central section, plus a fractional shunt capacitance KC, assoicated with one of the fractional end sections. Therefore,

C5=Cs=0.5C'+KC (3) For the best impedance match over a. wide frequency range, K should, in general, have a value falling between 0.3 and 0.4,,with approximately 0.36 as the optimum value. When K is equal to 0.36, from Equation 3 Cs=Ce=0.5C+0.36C=Q.86C and therefore,

From Equation 1 CBRI 0.12

mi-ms and from Equation 2 L1=Ls=0.36L3 (7) The cut-off must be above the upper limit of the signal range, and is preferably several times that frequency. The higher the cut-of! frequency Ic the better the impedance match will be over the signal range. It is seen from Equation -8 that J0 is inversely proportional to the magnitude of the shunt capacitance C5 or Go. It is, therefore, desirable to keep these capacitances as small as is consistent with a high balance to ground and close coupling between the line windings of the suppression coil.

The two line windings of each of the auxiliary coils L1, La and L may be constructed as two duolateral solenoids on a fiber tube. Stranded wire is preferably used, to reduce the attenuation introduced into the loop circuit. Furthermore, they may be inductively coupled, for the same reason, in which case they are connected in the series-aiding relationship for the loop circuit. These coils are preferably made variable in inductance so that the image impedance of the filter may be precisely adjusted to the desired nominal value. Ordinarily, an adjustment range of about eight or ten per cent in. the series-aiding inductance will be suflicient. Plug-type cores of powdered magnetic material inserted in the tube may conveniently be used for this purpose, provided certain requirements are met. The two cores associated with the two line windings of each coil are preferably placed on a single push rod so that the self-inductance of each line winding will be changed by the same amount, thus maintaining the balance to ground. Since these windings may carry signal currents at high energy levels it is necessary to limit the flux density in the cores to such a value that harmonic generation is negligible.

What is claimed is: A

1. A wave transmission network for suppressing undesired longitudinal currents over a-range of frequencies while freely transmitting signal currents in the same range in the loop circuit of a transmission line comprising two suppression coil sections and three auxiliary coils all connected in series, each of said sections and each of "said auxiliary coils comprising two equal line windings connected one in each side of the line, said two windings of each of said sections having close inductive coupling and being connected in the series-opposing relationship for said loop circuit, one of said auxiliary coils being connected becapacitances of said capacitors being proportioned to cause said sections to antiresonate at a, frequency near the upper end of saidrange of frequencies.

7. A network in accordance with claim 1' in which the inductances of said auxiliary coils are variable. 1 i

8. A network in accordance with claim 1 in which K is approximately equal to 0.36.

.9. A network in accordance with claim 1 in which K is approximately equal to 0.36 and the ratio of the inductance of one of said other auxiliary coils to the inductance of said one auxiliary coil falls between 0.3 and 0.4.

10. A wave transmission network for supressing undesired longitudinal currents overa range of frequencies while freely transmitting signal currents in' the same range in the loop circuit of atransmission line comprising two suppression coil sections and three auxiliary coils all connected in series, each of said sections and each of said auxiliary coils comprising two equal line windings connected one in each side of the line, said two windings of each of said sections having close inductivlecoupling and being connected in the series-opposing relationship for said loop circuit,pone of said auxiliary coils being tween said sections, the other of said auxiliary coils being connected, respectively, at the outer ends of said sections and the inductance of said 'the square of the impedance of a terminal load with which said network is to o erate and divided by'the factor (0.5+K) when K has avalue fallins between 0.3 and 0.4.

2. A network in accordance with claim 1 in which the ratio of the inductance of one of said other auxiliary coils to the inductance of said one auxiliary coil is approximately equal to K.

3. A network in accordance with claim 1 in which K is approximately equal to 0.36 and the ratio of the inductance of one of said other auxiliary coils to the inductance of said one auxiliary coil is approximately equal to K. n

4. A network in accordance with claim 1 in which said line windings of each of said auxiliary coils are inductively coupled and are connected in the series-aiding relationship for said loop circuit.

5. A network in accordance with claim 1 in which said suppression coil sections antiresonate at a frequency near the upper end of said range of frequencies.

6. A network in accordance with claim 1 which includes two capacitors, one of said capacitors being connected to bridge said line windings in one side of the line associated with said sections, the other of said capacitors being connected to bridge said line windings in the other side of the line associated with said sections and the connected between said sections, the other of said auxiliary coils being connected, respectively, at the outer ends of said sections and the inductances of said auxiliary coils being proportioned with respect to the shunt capacitance effective between said line windings of said sections to provide a low-pass wave filter effective in said loop circuit. I

11. A network in accordance with claim 11 in which said filter has its cut-off at a frequency which is several times that of the upper limit of saidv range of frequencies.

12. A network in accordance with claim 10 in which said filter has a nominal image impedance at one end substantially equal to the impedance of the load with which said filter is to operate at said one end.

13. A network in accordance with claim 10 in which the inductance of said one auxiliary coils is approximately equal to the shunt capacitance eilective between said line windings of one of said sections multiplied by the square of the impedance of a terminal load with which said network is to operate and divided by 0.86.

14. A network in accordance with claim 10 in which the ratio of the inductance of one of said other auxiliary coils to the inductance of said one auxiliary coil falls between 0.3 and 0.4.

15. A network in accordance with claim 10 in which the inductance of one ofsaid other auxiliary coils is approximately equal to 0.36 of the inductance of said one auxiliary coil.

16. A network in accordance with claim 10 in which said line windings of each of said auxiliary coils are inductively coupled and are connected in the series-aiding relationship for said loop circuit.

17. A network in accordance with claim 10 in which said suppression coil sections antiresonate at a frequency near the upper end of said range of frequencies.

18. A network in accordance with claim 10 which includes two capacitors, one of said canacitors being connected to bridge said line windings in one side of the line associated with said sections, the other of said capacitors bethe other side of the line associated with said sections and the capacitances-of said capacitors being proportioned to cause-said sections toantiresonate at a frequency near the upper end of said range of frequencies.

19. A .network in .accordance with claim .10

in which the inductances of :said auxiliary coils are variable.

20. A network in accordance with claim '10 in whichsaid filter has itscut-oif at a frequency which is several times that of :the upper limit ofsaid rangeof frequencieszand a nominal image-impedance at oneend substantially equal to .the impedance of .the load with which said filter is to operate at said one end.

. .21. A network in accordance with claim 10 in'whichsaidfilter has its cut-off at a frequency which is severaltimes'that of theaupper limit of said range of frequencies and the ratio of the inductance of one of said other auxiliary coils to the inductance of zsaid-one auxiliary coil falls between 0.3 and A.

v22. A network in accordance with claim in which said filter has its cut-off at a frequency which is severaltimes that of the upper limit of said range of frequenciesandsaid suppression coil sections antiresonate at a frequency near the upper end of said range of frequencies.

23. A network in accordance with claim 10 in which said filter has a nominal image impedance .at one end substantially equal :to the impedance of the load with which said filter is to operate at said one-end and the ratioof the inductance of one ofsaidother auxiliary coils to the inductance of said one auxiliary coil falls between 0.3 and 24. A network in .accordance with claim 10 in which said filter has a nominal image impedance at oneend substantially equal to the impedance of the load with which said filter is to operate at said one end and said suppression coil sections anti-resonate at a frequency near the upper end of said range of frequencies.

25. A network in accordance with claim 10 in which the ratio of the inductance of one of said other auxiliary coils to the inductance of said one auxiliary coil falls between 0.3 and 0.4 and said suppression coil sections antiresonate at a frequency near the upper end of said range of frequencies.

26. A network in accordance with claim 10 in which said filter has its cut-off at a frequency which is several times that of the upper limit of said range of frequencies, said filter has a nominal image impedance at one end substantially,

equal to the impedance of the load with which said filter is to operate at said end and the ratio of the inductance of one of said other auxiliary equal to the impedance of the load with which proximately equal to 0.36 of the inductance of said filter is to operate at said end and said suppression coil sections antiresonate at a frequency near the upper end of saidrange of frequencies.

28. A network in accordance with claim 10 in which said filter has its cut-off at a frequency which is several times that of the upper limit of said range of frequencies, the ratio of the in-' ductance of one of said other auxiliary coils to the inductance of said one auxiliary coil falls between 0.3 and 0.4 and said suppression-coil sections antiresonate at a frequency near the upper end of said range of frequencies.

29. A network in accordance with claim 10 in which said filter has a nominal image impedance at one end substantially equal to theimpedance of the load with which said filter is to operate at said one end, the ratio of the inductance of one of said other auxiliary coils to the inductance of said one auxiliary coil falls between 0.3 and 0.4

and said suppression coil sections antiresonate at a frequency near the upper end of said range of frequencies.

30. A network in accordance with claim 10 in which the inductance of said one auxiliary coil is approximately equal to the shunt capacitance effective between said line windings of one of said sections multiplied by the square of the im pedance of a terminal load with which said network is to operate and divided by 0.86, and the ratio of the inductance of oneof said other auxiliary coils to the inductance of said one auxiliary coil falls between 0.3 and 0.4.

31. A network in accordance with claim 10 in which the inductance of said one auxiliary coil is approximately equal to the shunt capacitance effective between said line windings of one of said sections multiplied by the square of the impedance of a terminal load with which said network is to operate and divided by 0.86, and the inductance of one of said other auxiliary coils is apsaid one auxiliary coil.

STUART G. HALE. 

